A solid electrolyte is free from liquid leakage and is thereby safe, and is highly reliable, and thus it is used as a material of e.g. a cell, a gas sensor and a double layer capacitor. As a material for forming a solid electrolyte, a lithium type material has been widely used. Further, as a cathode of a non-aqueous electrolyte secondary cell, lithium cobalt oxide and its composite oxide have been employed. A secondary cell employing e.g. lithium cobalt oxide as a cathode has a high operating voltage and a high energy density and is light in weight, and thus it is widely used as a power source of a portable electronic device, an electric power tool and an electric automobile.
However, the material such as lithium cobalt oxide employs a rare element in view of the resource and is thereby expensive and in addition, it has drawbacks such that its safety is not necessarily sufficient. Further, for a non-aqueous electrolyte secondary cell, higher performance and larger capacity are required. Under these circumstances, development of a material which is available at a low cost, is safe and has excellent performance, which will replace lithium cobalt oxide, is in an urgent need. As such an alternative material, in recent years, NASICON type or olivine type lithium iron phosphate material attracts attention, and its development and research are remarkably in progress.
As a method for producing a lithium iron phosphate type material, various methods have been proposed. Patent Document 1 discloses, as a method for producing NASICON type lithium iron phosphate, a method of producing LinFe2(PO4)3 (0<n≦3) by solid phase reaction. In this method, after lithium iron phosphate is formed by solid phase reaction, in order to use a lithium iron phosphate type active material as a cathode material, the lithium iron phosphate should be pulverized. Therefore, the cell performance when such lithium iron phosphate is employed for the cathode may be deteriorated e.g. due to a decrease in the crystallinity of such lithium iron phosphate.
Patent Document 2 discloses a method of melting a material containing e.g. Li2O, Fe2O3, P2O5 and Nb2O5, quenching the resulting melt to form precursor glass, and subjecting this precursor glass to heat treatment to produce NASICON type, olivine type or spinel type lithium iron phosphate. To form the precursor glass, e.g. a Nb2O5 source is added in addition to a Li2O source, a Fe2O3 source and a P2O5 source. Accordingly, only lithium iron phosphate crystals containing e.g. Nb derived from the starting material, such as Li(Fe,Nb)(PO4)3 or Li3(Fe,Nb)2(PO4)3 has been obtained.
Patent Document 2 failed to disclose a method for producing lithium iron phosphate particles. Further, although Patent Document 2 discloses a power X-ray diffraction pattern of lithium iron phosphate, in order to obtain lithium iron phosphate particles, a formed product having precursor glass crystallized by heat treatment should be pulverized, which may lead to various drawbacks such as a decrease in the crystallinity of lithium iron phosphate. Further, in a case where laser treatment is employed for heating, e.g. a cost increase in the crystallization step is inevitable.
As described above, lithium iron phosphate is available at a low cost and is highly safe as compared with lithium cobalt oxide, and thus it is expected as a material for a solid electrolyte and a cathode-material for a secondary cell. Particularly, NASICON type lithium iron phosphate represented by LinFe2(PO4)3 (0<n≦3), which contributes to an improvement in properties of a solid electrolyte or a cathode for a secondary cell, attracts attention. However, a conventional method for producing lithium iron phosphate particles has drawbacks such as low crystallinity of particles to be obtained, or such that the uniformity of the particle size or the chemical composition is likely to be decreased.